The Ahriman is an enemy from Final Fantasy XIII.
They only appear when an Imp calls them into battle, and are more powerful than said enemy. The player can avoid engaging the Ahriman by defeating the Imps as quickly as possible. As they award little CP and drop spoils that are readily available elsewhere, there is no reason to fight them apart from completing the Datalog.
If the player is looking to fight an Ahriman, they can try engaging the group of three Imps in front of the treasure orb and wait for them to conjure the Ahriman.